1. Software and Algorithms
Chapter 3
Software and Algorithms
Dr. Khaled Wassif
Fall
CS111

2. CHAPTER OUTLINE What is Software? Programming Languages
Why using Algorithms?
What is an Algorithm?
Representing Algorithms
Operations of Algorithms
Input / Output
Computations
If-else-endif
While-endwhile
CS111 Introduction to Computers By Dr. Khaled Wassif

3. What is Software (S/W)?
S/W consists of a set of machine-readable instructions, called programs, that cause the computer to perform a task.
There are two basic types of software:
Applications software:
Designed to satisfy a user’s specific needs.
May be custom or packaged (off-the-shelf).
System software:
Without it H/W and applications S/W are useless.
Operating System – Coordinate all hardware components.
Compilers – Translate source program into object program.
CS111 Introduction to Computers By Dr. Khaled Wassif

4. Programming Languages
Enables a programmer to write programs that are more or less independent of a particular type of computer.
Considered high-level languages because they are closer to human languages and further from machine languages.
Generations of computer languages are:
First generation: machine language
Second generation: assembly language (low-level language)
Third generation: high-level programming languages, as C, C++, and Java.
Forth generation: query languages; used to access databases, as SQL
Fifth generation: artificial intelligence languages, as Prolog, and Lisp
CS111 Introduction to Computers
By Dr. Khaled Wassif

5. Algorithms
An algorithm is a step by step method for solving a problem.
We use algorithms all the time in our daily life, for example:
Cooking recipes
Directions how to get to places
Performing mathematical tasks such as:
Calculate the students' GPA
Calculate the interests for invested money in a bank
An algorithm for calculating the area of a square:
Step 1. Get the value of Side
Step 2. Area = Side * Side
Step 3. Print the value of Area
CS111 Introduction to Computers
By Dr. Khaled Wassif

6. History of Algorithms
Named after the Persian mathematician Muhammad Ibn Musa Al-Khwarizmi ( in Khwarizm, Uzbekistan) developed a strategy for calculating heritage proportions using algebraic methods.
His name was turned into Algorism and that evolved Algorithm.
CS111 Introduction to Computers
By Dr. Khaled Wassif

7. History of Algorithms (cont.)
The oldest known algorithm is probably Euclid's Algorithm to determine the greatest common divisor (GCD) of two integers (circa B.C).
Method: To find the GCD of two numbers, repeatedly replace the larger by subtracting the smaller from it until the two numbers are equal.
Only subtraction and comparison operations are needed.
Example: GCD of 132 and 168
36 132
36 96
36 60
36 24
12 24
12 12
So the GCD of 132 and 168 is 12.
CS111 Introduction to Computers
By Dr. Khaled Wassif

8. Why Using Algorithms?
If we can specify an algorithm to solve a problem, then we can automate its solution.
Why use a computer?
Computers are fast:
They can perform operations without errors at speed unattainable by human beings.
They can store very large amount of information:
Human beings have a difficulty managing and keeping track of a large number of objects.
They are not task specific:
They can be programmed to perform different tasks. Most other tools can do only one thing.
Their tasks can be automated:
Computers are excellent at performing the same task over and over again on similar pieces of data.
CS111 Introduction to Computers
By Dr. Khaled Wassif

9. What is an Algorithm?
An algorithm is a well-ordered collection of unambiguous and effectively computable operations that, when executed, produces a result and halts in a finite amount of time.
An algorithm is well-ordered:
Each step of the algorithm is executed in the order in which it is written, or else the order is clearly stated.
An algorithm is unambiguous:
It must be clearly stated, in terms that the computer understands
An algorithm is effectively computable:
It must be possible for the computer to perform the operation and produce a result
An algorithm must halt in a finite amount of time:
It must even if it would take centuries to finish
CS111 Introduction to Computers
By Dr. Khaled Wassif

10. Representing Algorithms
Algorithms can be expressed in any language, from natural languages to formal programming languages or others.
Natural Languages (e.g. English, French)
Formal Programming Languages (e.g. Java, C++)
Something else?
What language to use?
Expressive
Clear
Precise and unambiguous
CS111 Introduction to Computers
By Dr. Khaled Wassif

11. Representing Algorithms: Natural Languages
Example:
Given is a natural number n. Compute the sum of numbers from 1 to n.
Representation with Natural Language
Initially, set the value of the variable result to 0 and the value of the variable i to 1. When these initializations have been completed, begin looping until the value of the variable i becomes greater than n. First, add i to result. Then add 1 to i and begin the loop all over again.
Disadvantages:
Too verbose
Unstructured
Too rich in interpretation (ambiguous)
Imprecise
CS111 Introduction to Computers
By Dr. Khaled Wassif

12. Representing Algorithms: Formal Programming Languages
Example:
Given is a natural number n. Compute the sum of numbers from 1 to n.
Representation with Formal Programming Language (Java)
public class Sum {
public static void main(String[] args)
{ int result = 0;
int n = Integer.parseInt(args[0]);
int i = 1;
while (i <= n) {
result = result + i;
i = i + 1;}
System.outp.println(result);}}
Disadvantages:
Too many implementation details to worry about
Too rigid syntax
CS111 Introduction to Computers
By Dr. Khaled Wassif

13. Representing Algorithms: Pseudo-code
We need a compromise between the two:
Pseudo-code
Computer scientist use Pseudo-code to express algorithms:
English like constructs (or other natural language)
Modeled to look like statements in typical programming languages.
Not actually executed on computers.
Allows us to think out a program before writing the code for it.
CS111 Introduction to Computers
By Dr. Khaled Wassif

14. Pseudo-Code: Input/Output and Computation
Input operations:
Allow the computer to receive from the outside world data values to use in subsequent computations.
General Format: get value for variable
Output Operations:
Allow the computer to communicate results of the computations to the outside world.
General Format: print value of variable
print the message “text”
Computation:
Performs a computation and stores the result.
General format: set variable to expression
CS111 Introduction to Computers
By Dr. Khaled Wassif

15. Pseudo-Code: Variables
A variable is a named storage:
A value can be stored into it, overwriting the previous value.
Its value can be copied.
Examples:
get value for A
Get a value to the variable A.
set A to 3
The variable A holds the value 3 after its execution.
set A to A + 1
Add 1 to the value of A (A is now 4)
print value of A
Display the current value of A (4)
CS111 Introduction to Computers
By Dr. Khaled Wassif

16. Pseudo-Code: Writing an Algorithm
Example: Given the radius of circle, determine the area.
Decide on names for the objects in the problem and use them consistently, e.g. radius, area (we call them variables)
Use the following primitive operations:
Get a value (input: e.g. get value for radius)
Print a value or message
(output: e.g. print value of area or print the message “Hello”)
Set the value of an object (e.g. set Pi to 3.14)
Performs a computation and stores the result.
set area to Pi × radius × radius
Using arithmetic operations: e.g. +, -, ×, /.
Satisfy priorities and using parenthesis.
Step 1: get value for radius
Step 2: set Pi to 3.14
Step 3: set area to Pi × radius × radius
Step 4: print value of area
CS111 Introduction to Computers
By Dr. Khaled Wassif

17. Pseudo-Code: Not too Strict on Syntax
Pseudo-code is a kind of programming language without a rigid syntax.
For example:
We can write:
set A to B+C as
set value of A to B + C
Or write:
set A to 0
set B to 0
set A and B to 0
CS111 Introduction to Computers
By Dr. Khaled Wassif

18. Operations of Algorithms
Algorithms can be constructed by the following operations:
Sequential Operation
Conditional Operation
Iterative Operation
CS111 Introduction to Computers
By Dr. Khaled Wassif

19. Sequential Operations
Each step is performed in the order in which it is written.
Example 1: Given the radius of circle, determine the area and circumference.
Names for the objects:
Input: radius
Outputs: area, circumference
Algorithm in Pseudo-code:
get value for radius
set area to (radius * radius * 3.14)
print value of area
set circumference to (2 * radius * 3.14)
print value of circumference
CS111 Introduction to Computers
By Dr. Khaled Wassif

20. Sequential Operations (cont.)
Example 2: Algorithm for finding the average of three numbers.
get value for A, B, C
set Sum to A+B+C
set Average to Sum/3
print value of Average
Example 3: The distance between two points (x1; y1) and (x2; y2) can be calculated using the following equation:
d =
Write an algorithm that calculates the value of d.
get value for x1, y1, x2, y2
set d to SQRT((x2-x1) * (x2-x1) + (y2-y1) * (y2-y1))
print value of d 2 2
CS111 Introduction to Computers
By Dr. Khaled Wassif

21. Sequential Operations (cont.)
Example 4: write an algorithm to convert temperature from Celsius to Fahrenheit using the following formula:
Tf = 9/5 * Tc + 32
The algorithm:
get value for Tc
set Tf to 9/5 * Tc + 32
print the message “Temperature = ”
print value of Tf
print the message “ degrees Fahrenheit”
CS111 Introduction to Computers
By Dr. Khaled Wassif

22. Sequential Operations (cont.)
Example 5: Write an algorithm to determine the flying time between two cities given the distance between them and the average speed of the airplane.
The algorithm:
get value for distance, speed
set time to distance / speed
print the message “Flying time is ”
print value of time
CS111 Introduction to Computers
By Dr. Khaled Wassif

23. Conditional Operations
Each operation is a control operations that allow us to alter the normal sequential flow of control in an algorithm.
Let's say you are going out tomorrow.
You need to make a decision on what to wear depending on the weather forecast.
Therefore, your decision will be based on a certain condition:
Is the weather going to be rainy?
If the answer is yes then, wear the raincoat.
If the answer is no then no raincoat is needed.
CS111 Introduction to Computers
By Dr. Khaled Wassif

24. Conditional Operations (cont.)
We can express the last situation using if-statement in the following manner:
if (weather = "rainy") then
print the message "Wear rain coat"
else
print the message "Do not wear rain coat"
endif
General format: if (condition) then
statement(s)
else if (condition)
... … …
e1se
Optional
CS111 Introduction to Computers
By Dr. Khaled Wassif

25. Boolean Expressions Comparisons Composite Expressions - Logical AND
Applied to two expressions of compatible type and always yield a boolean result:
a < b (less than)
a <= b (less than or equal)
a = b (equals)
a > b (greater than)
a >= b (greater than or equal)
A != b (not equal)
Composite Expressions - Logical AND
(X AND Y) yields true only if both X and Y evaluate to true.
Composite Expressions - Logical OR
(X OR Y) yields true if either X or Y, or both are true.
CS111 Introduction to Computers
By Dr. Khaled Wassif

26. Conditional Operation Examples
Heart rate is usually calculated as the number of heart beats in one minute and expressed as "beats per minute" (bpm). When resting, the adult human heart beats at about 70 bpm.
Write an algorithm that gets heart rate as input and decide whether rate is normal or not.
The algorithm:
get value for Rate
if (Rate = 70) then
print the message "Normal Heart Rate"
else
print the message "Abnormal Heart Rate"
endif
CS111 Introduction to Computers
By Dr. Khaled Wassif

27. Conditional Operation Examples
Write an algorithm that decides whether a given number is positive or negative.
Inputs: number
Output: Number sign
The algorithm:
get value for number
if (number < 0) then
print the message "The number is negative"
else
print the message "The number is positive"
endif
CS111 Introduction to Computers
By Dr. Khaled Wassif

28. Conditional Operation Examples
Write an algorithm that inputs two numbers x and y, and computes and displays the division of x over y if the value of y is not zero. If y does have the value 0, then display the message "Unable to perform division".
The algorithm:
get value for x, y
if (y = 0) then
print the message " Unable to perform division"
else
set d to x/y
print value of d
endif
CS111 Introduction to Computers
By Dr. Khaled Wassif

29. Conditional Operation Examples
Write an algorithm that gets the temperature as input and decide whether the weather is cold, hot or nice based on the following criteria:
Temperature > 30 Hot weather
Temperature < 18 Cold weather
Otherwise Nice weather
Inputs: temperature (temp)
Output: Weather state
CS111 Introduction to Computers
By Dr. Khaled Wassif

30. Conditional Operation Examples
The algorithm:
get value for temp
if (temp > 30) then
print the message "Hot Weather"
else if (temp 18) then
print the message "Cold Weather"
else
print the message "Nice Weather"
endif
CS111 Introduction to Computers
By Dr. Khaled Wassif

31. Conditional Operation Examples
Write an algorithm that gets the values of starting account balance, annual interest rate and annual service charge. The algorithm includes the annual service charge only if the starting account balance is less than 1,000 pounds at the beginning of the year. The algorithm should compute and display the balance after one year.
The algorithm:
get value for SB, IR, SC
if (SB < 1000) then
set EB to SB * (1 + IR) - SC
else
set EB to SB * (1 + IR)
endif
print value of EB
CS111 Introduction to Computers
By Dr. Khaled Wassif

32. Conditional Operation Examples
Write an algorithm that outputs the grade of the student according to the following rules:
Grade "A" if 90 ≤ mark ≤ 100
Grade "B" if 80 ≤ mark ≤ 89
Grade "C" if 65 ≤ mark ≤ 79
Grade "D" if 50 ≤ mark ≤ 64
Grade "F" if mark < 50
Inputs: mark
Output: grade
CS111 Introduction to Computers
By Dr. Khaled Wassif

33. Conditional Operation Examples
The algorithm:
get value for mark
if (mark >= 90) then
set grade to “A”
else if (mark >= 80) then
set grade to “B”
else if (mark >= 65) then
set grade to “C”
else if (mark >= 50) then
set grade to “D”
else
set grade to “F”
endif
print value of grade
CS111 Introduction to Computers
By Dr. Khaled Wassif

34. Conditional Operation Examples
Write an algorithm that outputs the maximum of three numbers.
The algorithm:
get value for A, B, C
if (A >= B) then
if (A >= C)
print value of A
else
print value of C
endif
if (B >= C)
print value of B Or
get value for A, B, C
if (A >= B and A >= C) then
print value of A
else if (B >= A and B >= C) then
print value of B
else
print value of C
endif
CS111 Introduction to Computers
By Dr. Khaled Wassif

35. Iterative Operations
Allow the repetition of a block of statements according to a condition.
Example:
Printing numbers from one to ten.
Iteration is sometimes called looping.
Common style of looping is using while statement.
General format: while (condition)
statement(s)
endwhile
The purpose of the while loop is to repeat a block of statements while the condition is true.
CS111 Introduction to Computers
By Dr. Khaled Wassif

36. Iterative Operation Examples
Write an algorithm to print numbers from one to ten.
Loop: initial value: 1
final value: 10
step: 1
The algorithm:
set num to 1
while (num <= 10)
print value of num
set num to num + 1
endwhile
Initial value
Final value
Step
CS111 Introduction to Computers
By Dr. Khaled Wassif

37. Iterative Operation Examples
Modify the previous algorithm to print numbers from one to n.
Loop: initial value: 1
final value: n
step: 1
The algorithm:
get value for n
set num to 1
while (num <= n)
print value of num
set num to num + 1
endwhile
CS111 Introduction to Computers
By Dr. Khaled Wassif

38. Iterative Operation Examples
Write an algorithm to print sum of numbers from one to n.
The algorithm:
get value for n
set sum to 0
set num to 1
while (num <= n)
set sum to sum + num
set num to num + 1
endwhile
print value of sum
CS111 Introduction to Computers
By Dr. Khaled Wassif

39. Iterative Operation Examples
Write an algorithm to compute and print the factorial of n.
The algorithm:
get value for n
set fact to 1
set num to n
while (num >= 1)
set fact to fact * num
set num to num - 1
endwhile
print value of fact
CS111 Introduction to Computers
By Dr. Khaled Wassif

40. Iterative Operation Examples
Write an algorithm that prints all the leap years from the year to the year A leap year is a one that is divisible by 4.
The algorithm:
set year to 1901
while (year <= 1999)
if (year % 4 = 0) then
print value of year
endif
set year to year + 1
endwhile
CS111 Introduction to Computers
By Dr. Khaled Wassif

41. Iterative Operation Examples
Given 800 kilograms of a radioactive substance that decreases in mass by half every year. Write an algorithm that calculates how many years it takes until it reaches a value of 25 kilograms.
The algorithm:
set mass to 800
set years to 0
while (mass > 25)
set mass to mass/2
set years to years + 1
endwhile
print value of years
CS111 Introduction to Computers
By Dr. Khaled Wassif

42. Iterative Operation Examples
Euclidean algorithm finds the greatest common divisor of two positive numbers X and Y by repeatedly replacing the larger number with the result of subtracting the smaller one from it until the two numbers become equal. Express this algorithm.
The algorithm:
get value for X, Y
while (X != Y)
if (X > Y) then
set X to X - Y
else
set Y to Y – X
endif
endwhile
print value of X
CS111 Introduction to Computers
By Dr. Khaled Wassif